By Ron B. Davis Jr., Georgetown University
For atoms, the volume they occupy is based on the outermost layer of electrons. The relative sizes of atoms and ions can have an impact on how they arrange themselves in solids and on their bonding behavior as well. So, the overall volume occupied by an atom of a given element can be a very important clue to its chemistry.
Size of an Atom
Atoms are incredibly small in size, from the human perspective. Typical atoms have a radius somewhere on the order of about 100 picometers. This is one hundred trillionths of a meter. One trillion atoms per centimeter.
To put that in perspective, for a typical-sized atom like tin, a square array of atoms, one million on a side, would fit on the head of a pin.
Yet, as small as atoms are, their relative size from one element to another does vary, and that can have important consequences when it comes to how each element behaves. Everything from physical properties, such as boiling points, to chemical properties, including the size, shape, and even reactivity of the molecules that make up compounds, can depend on the size of atoms.
The element with the smallest atomic radius is helium (not hydrogen), measuring just under 50 picometers. At the diagonal opposite corner of the periodic table, the element with the largest atomic radius is Francium, about six times larger, at about 300 picometers.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
X-Ray Crystallography
In the 1920s, just as the now-standard version of the periodic table was taking hold, x-ray crystallography allowed scientists to measure the radius of atoms for the first time.
By shooting high-energy photons through crystalline materials and measuring the scattering patterns produced as the photons exited the sample, the size of the atoms within the sample could be estimated.
These results showed conclusively that the volume taken up by different kinds of atoms can be quite different, but also follow a well-defined trend, with radius decreasing as we move from left to right and bottom to top on the periodic table.
This was because, as we move left to right across a row, the mass of the atom increases, while the atomic radius actually decreases.
Atomic Radii of Elements
In order to understand this, we need to take a look at the atomic radii of some elements and think about how the periodic table can be a guide for us in how these radii trend.
Let’s take the example of a few noble gases such as helium, neon, argon and krypton. Looking at their electron configurations, we can see how they differ. Helium, of course, is a 1s2, neon has a second energy level, argon has a third, and krypton has a fourth.
Valence Shell
From the atomic radius perspective, this means that as we step down through the periodic table, we have to think about which valence shell is operative. In the case of helium, for example, we’ve got only one shell of electrons.
But, as we step down to neon, we add an entire second shell of electrons. Neon’s valence shell is the second shell. Argon’s is the third, and krypton’s is the fourth.
Trend of Decreasing Atomic Radius
So, each time we step down a column, we’re adding a huge additional shell of electrons, thereby increasing the radius of that atom significantly, in spite of the fact that we’re increasing nuclear charge.
Hence, we see a trend of decreasing atomic radius from bottom to top within a column. It all does seem fairly intuitive. Less massive atoms have smaller radii within the column.
Second Row of the Periodic Table
And, yet, when we turn our attention to the trend across rows, we see something a little bit different.
From the second row, elements such as lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon, the electron configurations do not differ in the same way that the electron configurations within a column does. Naturally, they’re all different, but in this case, they all have the same valence shell, the second energy level.
This means that even as we’re adding electrons moving across the road, we’re adding those electrons to the same shell, which isn’t significantly contributing to a change in atomic radius.
The Nuclear Charge
What is contributing significantly to a change in atomic radius is the nuclear charge. As we know, lithium has three protons, whereas neon at the end of the series has 10.
So, if we think about that, having a nuclear charge of plus three for lithium, plus four for beryllium, plus five for boron and so on, means that as we move left to right, those nuclei are progressively pulling harder and harder on electrons within the same energy shell.
Greater Contraction in Size
This is why we see a greater contraction in size, meaning a decreasing atomic radius as we step from left to right. Even though the atoms themselves are becoming more massive, their radii are actually progressively shrinking as we move across the table in this way.
Thus, if we combine these two trends, we get an overall trend throughout the table in which we find the largest atoms tend to be in the bottom left corner, with the smallest in the top right region of the table. Clearly, a physical property as basic as the size of the atom exhibits some remarkable trends across the entire periodic table.
Common Questions about the Periodic Table and the Size of an Atom
Typical atoms have a radius somewhere on the order of about 100 picometers. This is one hundred trillionths of a meter.
In the 1920s, just as the now-standard version of the periodic table was taking hold, x-ray crystallography allowed scientists to measure the radius of atoms for the first time.
We see a trend of decreasing atomic radius from bottom to top within a column. It all does seem fairly intuitive. Less massive atoms have smaller radii within the column.
